1. Field of the Invention
The present invention relates to a waterglass precipitate recovery process for metals. More specifically, the process introduces waterglass to a process waste stream, the waterglass forms a matrix sludge with the metals in the stream, the waterglass matrix sludge formed therefrom is separated from the process stream and is then dissolved, freeing the metals dissolved in the waterglass matrix sludge for recovery. The waterglass may then be recycled for reuse and the metal may be further processed.
2. The Prior Art
Waterglass processes have been used to remove the final traces of uranium from waste streams produced by ammonium diuranate (ADU) and direct conversion (IDR) processes.
U.S. Pat. No. 4,349,513 discloses a process for recovering uranium in a liquid which adds waterglass to the liquid which forms a precipitate which captures the uranium and treating the precipitate with acid. The leached uranium is recovered as an acid solution. The precipitate is regenerated to waterglass by treatment with an alkali metal hydroxide solution. (See FIG. 1(A).)
A currently used uranium recovery waterglass process based on U.S. Pat. No. 4,349,513 adds a 6 w/o solution of sodium silicate (waterglass) to the process waste which is usually about 6 w/o ammonia, about 1 to 3 w/o fluorine, and about 15 ppm uranium in water. As used in this disclosure, the expression `w/o` is employed to mean `weight percent`. The waterglass precipitates, and forms a silicate fluoride matrix sludge which captures the uranium. The silicate fluoride sludge is filtered out of the process stream and sent to a holding tank.
Nitric acid is added intervally to reach a pH of between about 2 and 3 to solubilize the uranium as a weak solution of uranyl nitrate The sludge is then filtered off, solidified and disposed of in 55 gallon drums. The dilute uranyl nitrate solution is sent to solvent extraction where the uranium is recovered and purified. The uranium free nitrate solution is then boiled to recover nitric acid and the sludge is solidified and disposed by, for example, burial. The sludge in the uranyl nitrate solution is mainly silica which is dissolved during the nitric acid leach of the water glass sludge. (See FIG. 1(B).)
There are several disadvantages to the current waterglass processes. A large amount of the metal and/or uranium that is held in the waterglass sludge is lost with the silica since it is unfeasible to perform a good wash of the silica cake. The silica cake is difficult to filter due to the very fine particle size (&lt;5 .mu.m) of the cake. Secondly, as nitric acid is added to the waterglass sludge, localized areas of pH of less than 2 are formed. In these localized areas, the free fluoride in the cake then combines with the nitric acid to form hydrofluoric acid which in turn attacks and dissolves some of the silica. The dissolved silica in the metal and/or uranyl nitrate solution is sent back to the solvent extraction area which causes plugging by solids during the solvent extraction. Moreover, the sludge carries fluoride ion to the nitric acid evaporator which makes the resulting process stream highly corrosive As a result, the acid recovery apparatus must be frequently replaced
These prior art processes provide low concentrations of uranium in the waterglass leach liquid which results in very large volumes of uranyl nitrate which need to be processed during solvent extraction. Finally, the leached silica sludge currently produced by the modified process cannot be re-dissolved in concentrated caustic due to the non-reactive nature of the precipitate. Therefore, the silica sludge must be disposed of by burial.
There is a need, therefore, for an economical, simple waterglass precipitate recovery process which produces a higher concentration of fluoride-free metals in the metal nitrate stream, and would eliminate the need for burial of the processed waste.